Obsidian Artifacts Studies

Close-up detail of raw obsidian rock material

In 2011 the BLM Hollister Field Office began a comprehensive review of obsidian artifacts collected since the late 1970's. This review included determining the geologic source of the artifacts through x-ray fluorescence (XRF) analysis and measurement of hydration rims that are created over time as the artifact absorbs water from the environment.

Results

The majority of artifacts sampled came from prehistoric archeological sites located in Fresno and San Benito Counties. Preliminary results indicate that the predominant (or "preferred") source location that Native American California Indians utilized for obsidian stone tools was in present-day Inyo County at the Coso Mountains. The second most 'popular' source for obsidian to make artifacts also came from a trans-Sierran source called Casa Diablo located in present-day Mono County. The remaining 20-30% of the obsidian source material comes from the northern California coastal mountains in present day Lake and Napa Counties.

Obsidian studies in west-central California are important in understanding prehistoric archeological sites because there are no local sources of obsidian for people to quarry; this meant that people had to trade or travel over long distances (over 100 miles) to obtain this material. One reason obsidian was favored as a lithic (stone) material for making sharp tools like projectile points (arrowheads) is because of the glass-like qualities of obsidian rock.